I have a Pi 3 B, with 7" touchscreen, case and recommended split power cable. I am running the latest Picroft 8.8 which is built on Raspbian Jessie Lite.

After setting up all the above, everything has worked perfectly for 2 days. Today after several hours idle, I went to wake the screen up by simply pressing a key on the physical keyboard, and found that the screen is now black with fixed vertical coloured lines:

Power cycling, rebooting and shutting down have no effect on the screen. The Pi is working fine and responds to ssh / audio commands fine. The screen / pi / ribbon cable had not been touched or moved while the pi was idle.

Thanks - I will try with a clean raspbian install although this might be difficult to prove anything substantive - as mentioned the screen has worked fine with picroft for 2 days.

By FPC connector are you referring to the larger or the smaller of the two ribbons? The larger was already connected when I received the display from Pimoroni. The smaller one seems to be connected fine but I can certainly try removing and reseating again.

It's a while since I've run into this, but it could well be the larger ribbon cable- between the driver board and display- being improperly seated that's causing this problem. It's rare, but since they're factory assembled- presumably by hand- there's always the chance one might be loose.